ASHRAE.AC-02-1-3-2002 Compact Counterflow Gas Cool

THISPREPRINTISFORDISCUSSIONPURPOSESONLY,FORINCLUSIONINASHRAETRANSACTIONS2002,V.108,Pt.1.NottobereprintedinwholeorinpartwithoutwrittenpermissionoftheAmericanSocietyofHeating,RefrigeratingandAir-ConditioningEngineers,Inc.,1791TullieCircle,NE,Atlanta,GA30329.Opinions,findings,conclusions,orrecommendationsexpressedinthispaperarethoseoftheauthor(s)anddonotnecessarilyreflecttheviewsofASHRAE.WrittenquestionsandcommentsregardingthispapershouldbereceivedatASHRAEnolaterthanJanuary25,2002.ABSTRACTTheexittemperaturefromthegascoolerisanextremelyimportantparameteraffectingtheefficiencyofthetranscrit-icalair-conditioningandheatpumpcycle.Interestincarbondioxideasarefrigeranthasrecentlyledtothedevelopmentandtestingofseveralprototypes,allofcross-flowdesign.Thisarti-cleemploysasimulationmodeltoanalyzetrade-offs(pressuredropvs.heattransfer)involvedinapproachingcounter-flowconfigurationthroughmulti-slabconfigurations.Italsopresentsthefirstexperimentalconfirmationofthemodel.INTRODUCTIONAsanaturalandenvironmentallybenignrefrigerant,CO2(R-744)isattractingsignificantattention.SystemswithR-744aremorecommoninlaboratoriesandresearchanddevelop-mentcentersthaninreallife,atleastatthistime.Applicationscanrangefromautomotiveairconditioning,residentialairconditioning(a/c)andheatpumps,andwaterheaterstosomespecificapplications,suchasdrycleaning.Inmostair-conditioningoperatingranges,R-744systemsoperateintranscriticalmode.ThemajordifferencebetweentranscriticalandconventionaloperationisthatheatrejectionisinthesupercriticalregionbecausethecriticaltemperatureforR-744is31°C.Consequently,pressureandtemperaturearenotrelatedandthisopensadditionalcontrolandoptimizationissues.TranscriticalsystemswithR-744,inparticular,werenotoriousfortheirpoorperformance(Bhatti1997).NewlydevelopedsystemsdescribedinPettersenetal.(1997),aswellinpublicationsbyagroupsuchasYinetal.(1998),demon-stratedverygoodperformance.Thataffectsexpectationsandshowsincreasedstrengthoftheresearchactivity.Compressorsuctionanddischargepressures(3-12MPa)arefarhigherthanforconventionalhydrofluorocarbon(HFC)refrigerants.Thankstothep-trelationshipatthesepressures,higherpressuredropsmaybetoleratedintheheatexchangerswithoutsignificantlyimpairingcycleperformance.Moreover,highercompressorefficienciescanbeachievedbecauseofthelowerpressureratio.Heattransferisbetteralsobecauseoffavorablethermophysicalproperties.ThelogicalquestionishowtoimproveperformancefurtheroristherealimittotheCOP.Inthispaper,wewilltrytoaddressthefirstquestion,focusingongascoolerissues.Thispaperwilldiscusstheimportanceoftherefrigeranttemperatureatthegascoolerexitandtheninvestigatequan-titativelytwodistinctgascoolerdesignconcepts:themulti-passsingle-slabdesignsusedinexistingprototypesandthesingle-passmulti-slabcounterflowconfigurationsproposedhere.ThisideaisofutmostimportanceinthecounterflowarrangementforatranscriticalR-744systemconceivedin1998duringourearlierexperiments.IMPORTANCEOFTHEGASCOOLEREXITTEMPERATUREFigure1showsthepredictedrelationshipbetweenCOPanddischargepressurefordifferentgascoolerexittempera-turesandshowstheobtainablebenefitsfromacloserapproachtoambientairtemperature.Theevaporationtemperatureinthisexampleissetat3.9°C(correspondingtoevaporationpressure3.85MPa),andthesuctionlineinternalheatexchangereffectivenessis0.8.Pressuredropisneglectedtosimplifythepresentation.Thecompressorefficiencyusedinthecalculationwasfittedfromapreviousexperiment(Boeweetal.1999)at2000rpm.CompactCounterflowGasCoolerforR-744C.W.Bullard,Ph.D.J.M.Yin,Ph.D.P.S.Hrnjak,Ph.D.FellowASHRAEMemberASHRAEC.W.BullardandP.S.HrnjakareprofessorsattheUniversityofIllinois,Urabana,Ill.J.M.YinisatechnicaladvisoratModineManufac-turingCompany,Racine,Wisc.AC-02-1-32AC-02-1-3DespitetheassumptionsunderlyingFigure1,severalconclusionscanbedrawnfromit.First,whentheairinlettemperatureenteringthegascoolerincreases,theR-744exittemperaturealsoincreasessotheoperatingpressureneedstobeincreasedinordertomaximizecycleCOPintranscriticaloperation.ThisCOPdecreasesasthegascoolerexittemper-atureincreases.Second,theCOPcurvetendstobeflatterwhenthetemperatureatthegascoolerexitishigher.Third,theCOPcurveisnotsymmetricalongthepressureaxis.Atlowerexittemperatures,theCOPdropsmoresteeplyonthelow-pressuresidethanonthehigh-pressureside.Also,accuratehigh-sidepressurecontrolismoreimportantformaximizingCOPoncooldaysthanforthecaseofhigherinletairtemper-ature.Andfourth,whentherefrigeranttemperatureatthegascoolerexitislowerthanthecriticaltemperature(31°C),thereisnolocaloptimumCOP.ExperimentsconductedearlierwithaprototypeR-744system(Boeweetal.1999)showedthatforasingleslab,threepassgascooler,theapproachtemperaturedifferencebetweenrefrigerantexitandambientairrangedfrom2°Cto9°C.Ifthatdifferencecouldbereducedbyredesigningthegascooler(withoutviolatingpackagingconstraints),systemperfor-mancecouldbesignificantlyimproved,asshowninFigure1.Forexample,supposetheairinlettemperatureis34°C.Iftherefrigerantexittemperaturefromthegascooleris38°C,fromFigure1thehighestCOPisabout2.54,andthecorrespondingdischargepressureis9.28